Biocompatible gold nanoparticles (AuNPs) are preferably used as markers in living tissues. Meanwhile, anisotropic AuNPs have gathered great attention because they have a strong absorption band in the near-infrared (NIR) range in which the light can penetrate deeply into the human body. Moreover, when the anisotropic AuNPs can absorb the near-infrared light, they generate heat enough to kill cancer cells. Whereas gold nano-rods are one group of the typical anisotropic nanoparticles and are investigated as imaging systems and on cancer therapy, there are main issues on practical use of the nano-rods, namely, the messy synthesis process and their toxicity. To synthesize nano-rods, “seed particles” and a concentrated surfactant solution are needed, and the reaction takes long time (at least several hours). The surfactant used is toxic and hard to remove from the obtained gold nano-rods. These issues limit practical usages of the gold nano-rods. The other anisotropic nanoparticles also have similar problems for the multiple procedures and/or the toxicity of protecting agents.
Thus, there is a need for biocompatible gold nanoparticles with NIR absorption, and also a need for a concise process for synthesizing AuNPs.